Computational Insights into the Isomerism of Hexacoordinate Metal–Sarcophagine Complexes: The Relationship between Structure and Stability

Abstract

The hexacoordinate complexes that the macrobicyclic ligands {(NH3)2sar}2+ and {(NMe3)2sar}2+ (sar = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) form with transition metals such as CoIII, CoII and CuII can adopt several isomeric structures. In this article, we have firstly employed DFT methods to compute the relative stability of their Δ-ob3, Δ-ob2lel, Δ-lel2ob and Δ-lel3 isomers, as well as the activation barriers for their interconversion. In agreement with the experimental data, the results show that, in general, the different isomers of the CoIII and CoII complexes present similar free energies, whereas the CuII complexes show a strong tendency towards the lel3 form. In addition, the interplay between the structure and stability of these species has been studied by combining shape maps with a distortion/interaction energy analysis. In contrast to the geometries close to the ideal octahedron that all the studied Co complexes present, the lel3 structures of [Cu{(NH3)2sar}]4+ and [Cu{(NMe3)2sar}]4+ are better described as trigonal prisms. In such structures the ligand adopts a conformation significantly more stable than in the other isomers, and this drives the formation of lel3-[Cu{(NH3)2sar}]4+ and lel3-[Cu{(NMe3)2sar}]4+. Overall, the results show a clear relationship between the stability of a given isomer and its degree of distortion with respect to the ideal octahedron (or trigonal prism), with the latter being ultimately dependent on the transition metal and its radius.